The use of biomass for isolation of bio-oils, biochar and for energy production faces several problems due to, inter alia, its low energy density as a fuel.
The influence of chemical active additives on high temperature microwave pyrolysis, though not torrefaction, of biomass has been investigated just by few teams. For example, the effect of additives such as, sodium hydroxide (NaOH), sodium formate (HCOONa), sodium bicarbonate (NaHCO3), silicon dioxide (SiO2) and calcium hydroxide (Ca(OH)2) on the microwave processing of biomass has been studied by Barbara Krieger-Brockett in 19941 1 B. Krieger-Brockett, “Microwave pyrolysis of biomass”, Res. Chem. Intermed., 1994, 20, 39-49
M. Chen et al2 investigated eight inorganic additives (NaOH, Na2CO3, Na2SiO3, NaCl, TiO2, HZSM-5, H3PO4, Fe2(SO4)3) in terms of their catalytic effects on the pyrolysis. Experiments were carried out at ca. 470° C. under dynamic nitrogen atmosphere. It was found that all additives have significantly increased yields of solid products; however yield of liquid wasn't changed substantially. They also found that additives can change the composition of the organic products. Furfural and 4-methyl-2-methoxy-phenol have been identified as the two dominant organic components in the liquid products obtained from MW pyrolysis of acid (H3PO4 and Fe2(SO4)3) treated samples. Notably L-glucosan (levoglucosan) was not found or found only in trace amounts in the liquid products from pyrolysis of sawdust by microwave heating both in control microwave experiment and in the presence of inorganic additives. Furthermore, calorific values of bio-char and acid number of bio-oil have not been estimated. 2 Chena M., Wanga J., Zhanga M., Chena M., Zhub X., Mina F. and Tanc Z. “Catalytic effects of eight inorganic additives on pyrolysis of pine wood sawdust by microwave heating”, Journal Of Analytical And Applied Pyrolysis 2008, 82, 145-150.
Furthermore, conventional fuel furnaces, such as those used in coal fired power stations require the fuel to be pulverized or micronised. For example, coal for burning in a power station furnace will usually be pulverised to increase the fuel surface area. This also provides more efficient combustion, more complete burnout, lower emissions, and better heat liberation rates.
One particular disadvantage identified with the use of biomass in energy production is that whilst the biomass may be shredded is has thus far not been possible to achieve micronised particles sizes of biomass or biomass char which would be suitable for use in conventional furnaces and or for burning when admixed with conventional fuels, such as coal.
Several options are available for the valorisation of biomass for energy production although none can result in increasing the overall energy available from the biomass. The three core options are seen as                1. gasification, a high temperature process of degradation of biomass to produce gases which can then be used for energy production;        2. pyrolysis, a medium to high temperature process where the biomass is degraded to yield mainly oil products for energy production; and        3. torrefaction, a low temperature process for the production of solid fuels with an increased energy density than the original biomass.        
Under conventional heating conditions gasification is carried out between 750-1800° C., pyrolysis generally above 400° C. and torrefaction between 200 and 300° C.
In terms of method of production of an improved fuel, each of these has inherent problems. Gasification is an endothermic process and occurs at very high temperature so requires significant energy input to drive the process. The major product of pyrolysis processes is oil, with typical yields of 60-70%. The so-called “bio-oil” is easier to transport than the gas products of gasification but suffer reduced fuel efficiency due to high water content, phase separation and corrosive properties which make long-term storage an issue. Although torrefaction offers the lowest energy demand of all these processes, as it operates at the lowest temperature, and offers a product which burns cleanly there are several problems associated with the process.                In conventional processes difficulties arise in strict control of operating temperature, it is necessary to ensure the process does not go above 300° C.        The maximum achievable calorific value is usually much lower than that of coal, expected to be from 18 to 23 kJ/g (8000 to 10000 Btu/lb).2         The production costs are high.        
These problems have, along with a complex system, prevented torrefaction of biomass for fuel from practical commercial use. Various routes such as catalysis have been investigated in order to improve the process but still the process has not been optimised to a point where it can perform on a commercial scale. As a result of the problems faced in conventional torrefaction we have chosen to investigate microwave torrefaction as an easily controlled mobile technology and for increasing the energy density of biomass for use as fuel.
Microwave torrefaction, as defined here, has not been reported in literature. The following is a review of the current state of the art of microwave gasification and pyrolysis which are generally carried out at temperatures above 300° C.
Microwave irradiation is defined as “electromagnetic irradiation in the frequency range of 0.3 to 300 GHz”. Similar to domestic microwaves, specialised chemistry microwave reactors operate at 2.45 GHz. Microwave radiation is used in several applications including radar, communications, radiometry, medicine, physics, chemistry, and cooking food. Compared to conventional heating techniques a microwave process is advantageous in terms of shorter reaction times, higher heating efficiencies and greater control. The advantages of microwave technology in terms of mobility of small scale processors and value in waste reduction applications have been highlighted by Ruan et el.1 The use of microwaves is well established in many industrial and commercial applications. Microwave treatment of biomass is used before and or during extraction processes. The use of microwave for extraction of oil is gaining significantly in terms of commercial importance and wide spread utilization. It is employed in extraction of primary plant oils as well as extraction of petroleum oils from drilling muds, tar and sands. Microwave ovens have gained acceptance as a mild and controllable processing tool. Microwaves allow simple, rapid and low solvent consuming processes. Microwaves are already widely used for extraction of oil from solid to liquid (CEM-2005). The use of microwaves is also growing in the processing of materials. Microwave processing is now being used for “the production of advanced ceramics, the deposition of thermal barrier coatings, and the remediation of hazardous wastes” with new applications constantly being researched.
The use of microwaves for treatment of biomass is relatively new and dates back to the early 1970's. However, previous focus on microwave treatment of biomass has always been on high temperature pyrolysis, gasification and liquefaction of the starting material. Pyrolysis of biomass is usually conducted at temperatures above 350° C. Examples of substrates employed in this type of pyrolysis are numerous and include plant biomass such as wood or agricultural residues, plastics and municipal waste.
Low temperature pyrolysis, also known as torrefaction, is a thermal treatment usually carried out between 200 and 300° C. Torrefaction is a mild biomass pre-treatment process for upgrading the quality of biomass as fuel for combustion and gasification applications. Torrefaction can also be referred to as roasting, slow and mild pyrolysis and high temperature drying.
Conventional (non-microwave) torrefaction of biomass materials such as wood is well known. Torrefied wood has the moisture and most volatile organic compounds (VOC's) driven out resulting in a high percentage of carbon content. In addition, the chemistry and structure of the wood itself is converted into a new form by continued exposure to heat. Torrefaction of other biomass materials, such as those of plant origin, e.g. prunings and/or mown grass, or waste materials is also known.2 However, the micronisation or pulverisation of such torrefied materials has proved unsatisfactory.
Use of Additives
In typical microwave pyrolysis one of the key requirements is the presence of dielectric materials which are good absorbers of microwave energy and transfers heat directly. Examples of additives include inorganic oxides, carbons (e.g. carboniferous additives, such as graphite) and/or water. Biomass is largely transparent to microwaves, meaning that it is not easy to heat it directly. The use of absorbers overcomes this limitation and enables fast heating rates. In addition it is important to note that as pyrolysis proceeds, more carbon is generated from the biomass and the microwave heating becomes progressively more effective.
The use of additives was also investigated by F. Yu, R. Ruan et al.3 who employed char and NaOH and showed increased yields of oil and syn-gas respectively. They also observed variations in gas composition at elevated temperatures. 3 E.g. Jenkins B. M., Baxter L. L., Miles Jr. T. R., Miles T. R. Combustion Properties of Biomass. Fuel Proc Technol. 54 (1998) 17-46i
Microwave Gasification of Biomass
Miura et al4 and A. M. Sarotti et al5 looked at the optimisation of microwave pyrolysis of cellulose and wood for production of levoglucosan. They investigated particle size, power and irradiation time and source of biomass on the distribution of products in terms of water, oil, gas and char and they also conducted chemical analysis of their oil. However, neither investigated the calorific (heating) value of the wood based biomass or the biomass components. The temperature of the process was not directly controlled and was only determined as a result of the microwave power input and sample residence time. In our work, we have found that the control of the process temperature along with the power input was vital for the production of products with reproducible properties. 4 Evans R. J., Milne T. A. Molecular Characterisation of the Pyrolysis of Biomass. 1. Fundamentals. Energy & Fuels, 1 (1987) 123-1375 Nowakowski D. J., Jones J. M. Uncatalysed and potassium-catalysed pyrolysis of the cell-wall constituents of biomass and their model compounds. J. Anal. Appl. Pyrol. 83 (2008) 12-25.
Calorific value of microwave pyrolysis products of biomass have, however, been investigated by Menendez et al6 who have investigated the effects of microwave irradiation for pyrolysis above 500° C. They used microwave energy to pyrolyse sewage sludge at irradiation of 1000 W and a final reaction temperature of 1000° C., with the aim to optimise the fuel properties of the gas produced. They found that microwave pyrolysis produces more gas and less oil than conventional pyrolysis and additionally the amount of hydrogen in the gas mixture is much higher especially at temperatures 500° C. The calorific values of the collected fractions were up to 7, 37 and 10 kJ/g for char, oil and gas respectively. In comparison to typical values for petroleum derived fuels with 32, 42, 14 kJ/kg for coal, oil and gas these values are relatively lower. However although heating values were higher for oils their yield was relatively small, meaning that the greatest percentage of energy was accumulated in the gas fraction, with the oil fraction containing the lowest relative percentage of the energy. 6 Fahmi R., Bridgwater A. V., Thain S. C., Donnison I. S., Morris P. M. Yates N. Prediction of Klason lignin and lignin thermal degradation products by Py-GC/MS in a collection of Lolium and Festuca grasses. J. Anal. Appl. Pyrol. 80 (2007) 16-23.
They also tried to optimise the calorific value of char from biomass prepared between 500-1000° C. by microwave pyrolysis. They were unable to affect the calorific value with increased temperature. Moreover the calorific value of the char was similar to that prepared at the same temperature under conventional conditions (around 24 kJ/g).
Surprisingly, we have been able to produce char and oil fuel products from microwave torrefaction of biomass working at temperature between 120 and 300° C., resulting in a significant saving in energy input. The production of solid and oil fuel products is seen as favourable due to the ease of transport of the final product and their simple integration into current energy production systems. The decomposition of the biomass occurred at lower temperature and yielded a solid char of increased calorific value, comparable with coal.
Microwave Pyrolysis of Biomass for Liquefaction
Ruan et el1. and Heyerdahl et al. have investigated microwave biomass pyrolysis for the production of a liquid fuel. They worked at temperatures between 260-600° C., with optimum process conditions for liquefaction in the region of 350 and 600° C. Under these optimised conditions it was found that 50% of the biomass energy can be condensed into liquid products, just 20% in the char and the remaining 30% in the gas phase. The fuel properties of the char were not fully investigated. Analysis of the pyrolysis oil in terms of water content, density, pH, viscosity, elemental analysis and calorific value (CV) were reported. The properties reported, including calorific values, were in the range of similar bio-oils produced by conventional pyrolysis process and they were also similar to those disclosed in the present invention at around 19 kJ/g. However no discussion of how the properties of the oil product varied with processing conditions such as time, temperature, power or use of microwave activated additives was given. The authors reported that at elevated temperatures char yields could be decreased resulting in increased yields of hydrogen gas.
Microwave Pyrolysis of Biomass for Char Production
More recently, microwave induced pyrolysis has been investigated by Huang et al. They looked at pyrolysis of rice straw and have identified solid liquid and gaseous products. As they were unable to control temperature independently to power input, a calibration curve for a defined sample mass was prepared between for 50 W to 500 W, corresponding to a temperature interval of 105-563° C. The conditions for experimentation were set between 266-563° C. The calorific value of the rice straw solid residue results obtained was no more than 20 kJ/g, and no better than those achievable by conventional methods in this temperature range. They also found no correlation between the calorific value of the char and the microwave power/temperature. As a result of the poor calorific value of the char they investigated alternative applications for the solid, e.g. as absorbent (surface area up to 270 m2g−1). In our work we have found that it was possible, at temperatures lower than those reported, to prepare a char of higher calorific value, with the possibility to optimise the calorific value by control of microwave power and temperature. The char which we have produced is also physically different to those reported by Huang et al, with negligible measured surface area.
Although microwave pyrolysis of biomass above 300° C. from different sources has been investigated by other researchers the lower temperature process of microwave torrefaction for the production of a high calorific char has not been.
In microwave assisted torrefaction or pyrolysis of biomass a number of factors such as microwave power, reaction temperature and time, type of substrate and its moisture affect the efficiency of the process. However, as this disclosure demonstrates there may be additional factors which influence the efficiency of the process, for example, biomass densification and pre-treatment temperature.
These two aforementioned factors form the basis of the process disclosed in this invention and are a key advantage over existing microwave-assisted pyrolysis processes.
At a basic level density and size of biomass govern processing quantities, handling volumes and materials logistics. With increasing utilisation of biomass for renewable energy generation, densification techniques such as pelletisation and briquetting have become an essential prerequisite in biomass pre-processing. However, densification also influences the efficiency with which materials interact with microwaves. As demonstrated in Example 1.D densification enhances the efficiency of absorption of microwave energy and allows greater processing rates, which are essential in commercial application of this technology. Typically, simple densification techniques start with pulverisation of biomass followed by compacting the pulverised material into pellets, briquettes or continuous rods or logs. The compacting process involves pressing particles into desired shapes, for example, by means of a press wheel (pellets) or screw extrusion into a dye of determined shape. Heat is generated during the compression as a result of increasing pressure. This causes the lignin (plus other components) in the biomass to melt and/or flow to envelop and bind the particles in the desired final shapes. In some instances adhesives or adhesion promoting compounds may be added to affect the binding or particles may be heated in the process to cause the flow or setting of the binder. The resulting products, for example, pellets, briquettes or rods, exit the processor, or stage of processing, hot and it is this heat that is utilised in the process disclosed herein.
Biomass pre-heating in microwave-assisted pyrolysis of biomass is an important factor of this disclosure. A number of preceding inventions suggest preheating of biomass to minimise microwave power input.
For example, the disclosure of U.S. Pat. No. 5,084,141 suggests preheating organic material with a heated gas stream to temperatures above 250° C. and preferably between 300 and 500° C. However, this disclosure does not provide an informed explanation why these temperatures might be beneficial. Carbohydrate based biomass and, in particular, lignocellulosic plant biomass, does not interact well with microwaves to generate efficient heating. Many microwave absorber additives such as graphite have been explored in the patent and scientific literature to indirectly induce heating to non-absorbing substrates to overcome this shortfall. However, as this disclosure demonstrates, heating is an important pre-requisite to microwave-assisted biomass torrefaction.
At elevated temperatures, the main components of biomass interact more efficiently with microwave irradiation. At about 180° C. cellulose, typically the major component of plant biomass, undergoes a physical transition which increases the mobility of the cellulose polymer chains. This allows better charge transfer interactions between hydroxyl and/or other functionalities on the chains (or hydrogen-bonding network) and tends to increase electric conductivity. This in turn enhances interaction with the electromagnetic field of the microwave irradiation. Furthermore, it also affects the chemical reactions pathways involved in the decomposition of cellulose by altering the activation energies associated with charge transfer and other reactions as a result of proximity of functional groups in more mobile polymeric systems. This, coupled with the increased polarity of functional groups, is responsible for the different chemical composition of the resulting products observed in microwave-assisted processes compared to conventional pyrolysis systems. Similar physical changes occur for hemicellulose, but typically at lower temperatures of around 160° C. Physical transitions in lignin can occur at lower temperatures, but in a broader, less defined, manner. Additionally, the extended three dimensional structural network, as well as fewer hydroxyl functionalities, means that the activation effect is less pronounced for lignin.
Lignin, hemicellulose and cellulose are the main structural components of biomass, with the latter usually representing the largest proportion. In order to fully take advantage of the activation phenomenon, biomass should be preheated to around 180° C. At this temperature all structural components are activated and a relatively low power of microwaves and/or short irradiation time (within seconds) is required to affect the decomposition reactions and carbonisation. Furthermore, under these conditions the fast rates of reaction enable homogeneous decomposition of the samples. Lower temperatures are acceptable, but do not take full advantage of the activation phenomenon across all components and thus reaction rates are slower. The resulting chars are less homogeneous under comparable irradiation times with charring occurring from inside out. Furthermore, at lower temperatures fractions of volatiles are significantly lower and the calorific values of the resulting chars are poor. Higher pre-treatment temperatures are unnecessary and uneconomical, and too high a temperature in excess of 300° C. can cause premature thermal degradation and/or undesirable decomposition reaction pathways.
The process utilised in the present invention takes full advantage of the densification and preheating phenomena. It integrates two elements: densification and microwave torrefaction equipment in a continuous fashion.
The first element compresses the sample(s) to maximise throughput and/or the amount of material available for absorption of microwave energy. In the process the sample(s) are preheated due to mechanical friction and compression (and potentially additional chamber, wheel or screw heating, depending, inter alia, on the equipment design and type of biomass), but rather than dissipate that heat, as in conventional processes, the energy is directly utilised for the second element of the process: microwave decomposition. In this manner the first element not only provides densification, but also an efficient preheating method. Preheating has been described in other patents, such as, U.S. Pat. No. 5,084,141 for preheating biomass, but the use of gas, infrared or other methods requires additional energy and is less efficient, in particular for larger particles. Because the thermal diffusivity of biomass is poor these methods largely preheat the surface and not the core of the particle. In the invention disclosed herein heat is transferred to individual particles which are then compressed to the desired size/shape and, although a temperature gradient between the core and the surface is likely, it is still a more effective way of preheating the sample and it utilises the “waste” heat of the first process rather than requiring additional heating energy.
U.S. Pat. No. 7,101,464 discloses using heat generated from the pyrolysis process for preheating waste tyres prior to microwave pyrolysis, but this process does not integrate densification and is less applicable to biomass where (1) anaerobic conditions are required to suppress burning and thus large volumes of inert gas is needed; and (2) the exhaust gasses from pyrolysis, which contain valuable chemical products, can be lost or exposed to prolonged heating and microwave irradiation.
The importance of maintaining low temperatures during microwaving of biomass to produce chemically functional oils has been highlighted in U.S. Pat. No. 3,843,457 which employs low temperature microwave induced plasma under vacuum for pyrolysis of waste (largely carbohydrates). This helps to preserve chemical functionality and produce higher value chemical products. In the invention disclosed herein the processing temperature for the microwave assisted pyrolysis may be low, i.e. between 100 and 300° C. Additionally, the biomass may be fed into the microwave chamber through a narrow feed tube with one or more outlets for gas (FIG. 1) and liquid decomposition (FIG. 1) products. The outlets are placed at the point at which the decomposition reaction occurs enabling immediate removal of products from the microwave chamber thus preventing prolonged heating and exposure to microwaves.
A number of biomass microwave processor designs are reported in the patent literature. For example, U.S. Pat. No. 4,795,300 describes a processor where microwaves are used to dry the biomass prior to pyrolysis, but the pyrolysis itself is initiated with a powerful laser and occurs at temperatures in excess of 400° C. The process utilises a screw feed or an auger to mix and propel the materials through the process. Use of this material conveying method for biomass is the most popular in the existing designs. International Patent application No. WO 2006/057563 utilises an auger screw to transport materials and mix them to ensure even irradiation with microwaves which are irradiating the chamber from one side. A similar design is reported in US Patent application No. 2008/0063578 which describes generation of carbonaceous materials. In addition to mixing and propagation of the feed materials and products, an auger may enable contact heating of the material to be processed, as described in Canadian Patent No. 2,577,684.
An alternative method of feeding materials into and through the microwave chamber involves conveyor belts. U.S. Pat. Nos. 5,330,623 and 3,843,457 both employ a conveyor belt to transport organic materials through a microwave chamber. However these systems do not allow easy mixing of the material which might be unevenly irradiated. U.S. Pat. No. 4,631,380 discloses a design for sterilisation of grain whereby a number of microwave sources are arranged across and around a central cylindrical passage way thus irradiating the material evenly as it passes through. The material can be conveyed by an auger or a belt conveyor and the passage way is constructed using a microwave transparent material.
An alternative approach is proposed in International WO 2007/007068 whereby the whole chamber is rotated around its axis much like in a tumble dryer. The material is mixed and irradiated with microwaves through the chamber. This design is also equipped with vents in the inner wall of the chamber for venting gasses and the rotating action prevents the material from falling through these vents.
Open chambers or vents are used to enable the removal of pyrolysis vapours. However, the microwave processing chambers in these existing designs are typically large, leaving significant void spaces where vapours and/or residues can collect and continue to be heated and/or reacted. The vapours can react with themselves or the feedstock and condense on the walls of the chamber where they can further react to form heavier fractions such as tars. The deposits can potentially block microwave radiation windows/inlets affecting the irradiation of samples. Even with a flow of carrier gas or a vacuum, which is commonly employed, a large void volume is a disadvantage in that it allows secondary reaction after pyrolysis or torrefaction. Furthermore, larger microwave chambers will require larger amounts of inert gases and/or more powerful vacuum to (1) ensure the necessary anaerobic conditions are maintained throughout the pyrolysis/torrefaction reactions; and (2) ensure that the vapours are removed from the chamber.
Furthermore, fractionation of pyrolysis/torrefaction products according to the residence time of materials in the microwave is disclosed as part of the present invention. The system described herein enables fractionation of the liquid and gaseous products obtained from the degradation process. It may be controlled by altering outlet positioning, rate of feed, microwave power and/or positioning. This is a key advantage of the present invention over existing processors.
A similar principle can be utilised for additives which can be injected into the processor at different points to affect desired reactions. This may additionally enhance the flexibility and control of the process.
The present invention overcomes these disadvantages having a material carrier tube which is transparent to microwaves and closely surrounds the biomass material so as to reduce voids. Careful positioning of the outlet ports on the processor can help to remove gaseous, vaporous or liquid products as soon as they are formed. Additionally because of the close fit of the feed in the processor any char deposited on the inner walls of the processor can be wiped off/cleaned by the forward moving materials ensuring that the enclosure remains transparent to microwaves. Additionally, none of the prior known designs take advantage of the densification of biomass to maximise the absorption efficiency of the microwave energy nor the preheating generated through that process.
Densification of biomass can be achieved in a number of ways, for example: pelletising, briquetting or screw extrusion. A combination of screw extrusion and microwave irradiation is known and has been applied in food processing (DE3237267: Process for extruding a foodstuff with the use of microwaves) and extrusion of plastics and rubber articles (U.S. Pat. No. 1,493,836: Extrusion Process). However, it has only been used to warm or melt materials, but it has not been applied in preheating and/or pyrolysis/torrefaction of biomass.
European Patent Application No. 1978086 discloses a process for the production of biogas or bioethanol from water and lignocellulosic biomass in which an extruder is used to premix powdered lignocellulosic material and water to form a paste for microwaving, but this process involves hydrolysis and not pyrolysis/torrefaction of biomass and additionally does not benefit from densification and temperature increase, but merely from improved mixing between components.